Stripping is a term used in the polymer industry to describe removal of certain unreacted monomer and other volatile material from the mass of reacted, i.e. polymerized, material. The stripping occurs while the polymeric material is in an aqueous dispersion called a "latex". A stripping gas such as steam is mixed with the dispersion and retained in that condition long enough for substantial amounts of the volatile material from the polymeric material to be absorbed into the gas. Ideally, saturation equilibrium is reached during the dispersion-gas retention to maximize the stripping efficiency.
Stripping processes are particularly useful in separating unreacted styrene from styrene-butadiene polymer latex. The polymer exists as an emulsion of rubber particles in water phase and reference to an aqueous dispersion in this specification and claims means such an emulsion as commonly understood in the art of producing polymer latices. Whether the polymerization is to low conversion, e.g. 60-70%, or to high conversion, e.g. 95-98%, substantial amounts of unreacted monomer still remain within the product at the end of the polymerization. The monomer must be removed not only to provide a high quality polymeric product having no undesirable odor, degradation, and the like, but to attain greater economies by recovery of the monomer for reuse. Butadiene can be readily removed by flashing it off in a vacuum chamber because it is highly volatile. Styrene, on the other hand, having a boiling point higher than water, must be removed by stripping. Because of its relative inexpensiveness and compatibility with the polymerization chemistry, steam is usually and typically used as the stripping gas in stripping processes. Typically, the residual styrene in the polymer product must be reduced to levels of about 0.02-0.05% by weight for the polymer to be acceptable for commercial use.
The most direct method of stripping has been to distill the latex under reduced pressure. In this process, latex is introduced batchwise into a vacuum vessel and agitated to achieve good contact with steam which is introduced at the bottom of the vessel. Steam enriched with the volatile material is drawn off at the top of the vessel under vacuum. Due to the character of the dispersion, this process is very slow and difficult to operate. Excessive foaming and coagulation occurs making the process unsatisfactory for large scale commercial production because of the constant loss of polymer.
It has been proposed to make the distillation continuous while eliminating foaming and coagulation by simultaneously injecting latex and steam through an externally heated tube in a liquid-in-gas mixture. The steam is saturated with volatile material while the latex is partially evaporated along the length of the tube. Upon discharge from the heated tube, the volatile-laden steam is immediately separated from the latex by one or more flash and cyclone separators, see e.g. U.S. Pat. No. 2,467,769.
In this process, unrestricted passages through the heated tube are required to prohibit build-up of back pressure and in turn avoid coagulation and foaming. The latex is forced to flow along the walls of the tube at relatively high velocities so that boiling occurs along the walls of the tube without coagulation of the latex or formulation of a coagulated film on the heated surface. The turbulent high velocity flow also breaks up any bubbles which are produced within the tube and thus prevents the formation and build-up of foam. Although this process is operative, it is inefficient and commercially unsatisfactory. Very large amounts of steam are needed to strip a given amount of polymer, and large amounts of heat must be externally introduced into the system aside from the latent heat of the stripping gas and latex.
The most prevalently used commercial method of stripping is called "flash stripping". The flash stripper utilizes the principle of continuously mixing a stream of carrier gas, typically steam, with a stream of aqueous dispersion in such ratios that a liquid-in-gas or vaporous mixture is provided. The vaporous mixture is then continuously flashed into a vacuum chamber where the volatile-laden stripping gas evaporates and is drawn off separate from the dispersion. Successive stages may be employed to reduce the volatile material to the desired residual level.
The difficulty with flash stripping has been that it is only as good as the contacting step prior to flashing. Saturation equilibrium must be approached in the contacting step in as short a period of time as possible to provide a unit of commercial efficiency. Logically this is accomplished by free and unrestricted passage through a long tube with a minimum of pressure drop, while external heat is added for the formation of vapor. However, inordinate amounts of steam and an excessive number of stages are needed to reduce the residual contact of the volatile materials to an acceptable level with such a contacting technique.
It has been proposed to achieve a rapid and uniform vaporous mixture of the aqueous dispersion of polymeric substance in the stripping gas by restricting the flow through a tortuous path, see U.S. Pat. Nos. 3,073,380 and 3,469,617 to Palmason. That is, the dispersion-gas mixture is passed through a tortuous path of relatively narrow elongated crosssection at high velocity, while inputting heat to the mixture to effect volatilization during confinement. The velocity of the mixture through the tortuous path is maintained so that the disposition of coagulant upon the surfaces which define the passageways is inhibited and the mixture is discharged from the end of the tortuous path with the vapors above ambient temperature. The mixture is immediately injected into a vacuum chamber where the volatiles are flash evaporated and drawn off together with the stripping gas under vacuum.
Although providing improved efficiency when operating, the "tortuous path" strippers present serious difficulties in manufacture and in operation. Specifically, the tortuous path contacting section is expensive to build and therefore involves a relatively large capital outlay. Moreover, because of plugging in the tortuous pathways, it requires the operation to be shut down frequently and for relatively long periods of time while the contacting section is cleaned and regasketed. The reliability of the unit is thus low and the maintenance costs excessive. The efficiency of such stripping units can be increased by maintaining extra contacting sections which can be inserted into the system to keep the stripper in operation. But this further increases both the capital outlay and maintenance requirements.